<!doctype html>
<html>
<head>
<meta charset='UTF-8'><meta name='viewport' content='width=device-width initial-scale=1'>

<link href='https://fonts.loli.net/css?family=Open+Sans:400italic,700italic,700,400&subset=latin,latin-ext' rel='stylesheet' type='text/css' /><style type='text/css'>html {overflow-x: initial !important;}:root { --bg-color:#ffffff; --text-color:#333333; --select-text-bg-color:#B5D6FC; --select-text-font-color:auto; --monospace:"Lucida Console",Consolas,"Courier",monospace; --title-bar-height:20px; }
.mac-os-11 { --title-bar-height:28px; }
html { font-size: 14px; background-color: var(--bg-color); color: var(--text-color); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; -webkit-font-smoothing: antialiased; }
body { margin: 0px; padding: 0px; height: auto; bottom: 0px; top: 0px; left: 0px; right: 0px; font-size: 1rem; line-height: 1.42857; overflow-x: hidden; background: inherit; tab-size: 4; }
iframe { margin: auto; }
a.url { word-break: break-all; }
a:active, a:hover { outline: 0px; }
.in-text-selection, ::selection { text-shadow: none; background: var(--select-text-bg-color); color: var(--select-text-font-color); }
#write { margin: 0px auto; height: auto; width: inherit; word-break: normal; overflow-wrap: break-word; position: relative; white-space: normal; overflow-x: visible; padding-top: 36px; }
#write.first-line-indent p { text-indent: 2em; }
#write.first-line-indent li p, #write.first-line-indent p * { text-indent: 0px; }
#write.first-line-indent li { margin-left: 2em; }
.for-image #write { padding-left: 8px; padding-right: 8px; }
body.typora-export { padding-left: 30px; padding-right: 30px; }
.typora-export .footnote-line, .typora-export li, .typora-export p { white-space: pre-wrap; }
.typora-export .task-list-item input { pointer-events: none; }
@media screen and (max-width: 500px) {
  body.typora-export { padding-left: 0px; padding-right: 0px; }
  #write { padding-left: 20px; padding-right: 20px; }
  .CodeMirror-sizer { margin-left: 0px !important; }
  .CodeMirror-gutters { display: none !important; }
}
#write li > figure:last-child { margin-bottom: 0.5rem; }
#write ol, #write ul { position: relative; }
img { max-width: 100%; vertical-align: middle; image-orientation: from-image; }
button, input, select, textarea { color: inherit; font: inherit; }
input[type="checkbox"], input[type="radio"] { line-height: normal; padding: 0px; }
*, ::after, ::before { box-sizing: border-box; }
#write h1, #write h2, #write h3, #write h4, #write h5, #write h6, #write p, #write pre { width: inherit; }
#write h1, #write h2, #write h3, #write h4, #write h5, #write h6, #write p { position: relative; }
p { line-height: inherit; }
h1, h2, h3, h4, h5, h6 { break-after: avoid-page; break-inside: avoid; orphans: 4; }
p { orphans: 4; }
h1 { font-size: 2rem; }
h2 { font-size: 1.8rem; }
h3 { font-size: 1.6rem; }
h4 { font-size: 1.4rem; }
h5 { font-size: 1.2rem; }
h6 { font-size: 1rem; }
.md-math-block, .md-rawblock, h1, h2, h3, h4, h5, h6, p { margin-top: 1rem; margin-bottom: 1rem; }
.hidden { display: none; }
.md-blockmeta { color: rgb(204, 204, 204); font-weight: 700; font-style: italic; }
a { cursor: pointer; }
sup.md-footnote { padding: 2px 4px; background-color: rgba(238, 238, 238, 0.7); color: rgb(85, 85, 85); border-radius: 4px; cursor: pointer; }
sup.md-footnote a, sup.md-footnote a:hover { color: inherit; text-transform: inherit; text-decoration: inherit; }
#write input[type="checkbox"] { cursor: pointer; width: inherit; height: inherit; }
figure { overflow-x: auto; margin: 1.2em 0px; max-width: calc(100% + 16px); padding: 0px; }
figure > table { margin: 0px; }
tr { break-inside: avoid; break-after: auto; }
thead { display: table-header-group; }
table { border-collapse: collapse; border-spacing: 0px; width: 100%; overflow: auto; break-inside: auto; text-align: left; }
table.md-table td { min-width: 32px; }
.CodeMirror-gutters { border-right: 0px; background-color: inherit; }
.CodeMirror-linenumber { user-select: none; }
.CodeMirror { text-align: left; }
.CodeMirror-placeholder { opacity: 0.3; }
.CodeMirror pre { padding: 0px 4px; }
.CodeMirror-lines { padding: 0px; }
div.hr:focus { cursor: none; }
#write pre { white-space: pre-wrap; }
#write.fences-no-line-wrapping pre { white-space: pre; }
#write pre.ty-contain-cm { white-space: normal; }
.CodeMirror-gutters { margin-right: 4px; }
.md-fences { font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; overflow: visible; white-space: pre; background: inherit; position: relative !important; }
.md-fences-adv-panel { width: 100%; margin-top: 10px; text-align: center; padding-top: 0px; padding-bottom: 8px; overflow-x: auto; }
#write .md-fences.mock-cm { white-space: pre-wrap; }
.md-fences.md-fences-with-lineno { padding-left: 0px; }
#write.fences-no-line-wrapping .md-fences.mock-cm { white-space: pre; overflow-x: auto; }
.md-fences.mock-cm.md-fences-with-lineno { padding-left: 8px; }
.CodeMirror-line, twitterwidget { break-inside: avoid; }
.footnotes { opacity: 0.8; font-size: 0.9rem; margin-top: 1em; margin-bottom: 1em; }
.footnotes + .footnotes { margin-top: 0px; }
.md-reset { margin: 0px; padding: 0px; border: 0px; outline: 0px; vertical-align: top; background: 0px 0px; text-decoration: none; text-shadow: none; float: none; position: static; width: auto; height: auto; white-space: nowrap; cursor: inherit; -webkit-tap-highlight-color: transparent; line-height: normal; font-weight: 400; text-align: left; box-sizing: content-box; direction: ltr; }
li div { padding-top: 0px; }
blockquote { margin: 1rem 0px; }
li .mathjax-block, li p { margin: 0.5rem 0px; }
li blockquote { margin: 1rem 0px; }
li { margin: 0px; position: relative; }
blockquote > :last-child { margin-bottom: 0px; }
blockquote > :first-child, li > :first-child { margin-top: 0px; }
.footnotes-area { color: rgb(136, 136, 136); margin-top: 0.714rem; padding-bottom: 0.143rem; white-space: normal; }
#write .footnote-line { white-space: pre-wrap; }
@media print {
  body, html { border: 1px solid transparent; height: 99%; break-after: avoid; break-before: avoid; font-variant-ligatures: no-common-ligatures; }
  #write { margin-top: 0px; padding-top: 0px; border-color: transparent !important; }
  .typora-export * { -webkit-print-color-adjust: exact; }
  .typora-export #write { break-after: avoid; }
  .typora-export #write::after { height: 0px; }
  .is-mac table { break-inside: avoid; }
  .typora-export-show-outline .typora-export-sidebar { display: none; }
}
.footnote-line { margin-top: 0.714em; font-size: 0.7em; }
a img, img a { cursor: pointer; }
pre.md-meta-block { font-size: 0.8rem; min-height: 0.8rem; white-space: pre-wrap; background: rgb(204, 204, 204); display: block; overflow-x: hidden; }
p > .md-image:only-child:not(.md-img-error) img, p > img:only-child { display: block; margin: auto; }
#write.first-line-indent p > .md-image:only-child:not(.md-img-error) img { left: -2em; position: relative; }
p > .md-image:only-child { display: inline-block; width: 100%; }
#write .MathJax_Display { margin: 0.8em 0px 0px; }
.md-math-block { width: 100%; }
.md-math-block:not(:empty)::after { display: none; }
.MathJax_ref { fill: currentcolor; }
[contenteditable="true"]:active, [contenteditable="true"]:focus, [contenteditable="false"]:active, [contenteditable="false"]:focus { outline: 0px; box-shadow: none; }
.md-task-list-item { position: relative; list-style-type: none; }
.task-list-item.md-task-list-item { padding-left: 0px; }
.md-task-list-item > input { position: absolute; top: 0px; left: 0px; margin-left: -1.2em; margin-top: calc(1em - 10px); border: none; }
.math { font-size: 1rem; }
.md-toc { min-height: 3.58rem; position: relative; font-size: 0.9rem; border-radius: 10px; }
.md-toc-content { position: relative; margin-left: 0px; }
.md-toc-content::after, .md-toc::after { display: none; }
.md-toc-item { display: block; color: rgb(65, 131, 196); }
.md-toc-item a { text-decoration: none; }
.md-toc-inner:hover { text-decoration: underline; }
.md-toc-inner { display: inline-block; cursor: pointer; }
.md-toc-h1 .md-toc-inner { margin-left: 0px; font-weight: 700; }
.md-toc-h2 .md-toc-inner { margin-left: 2em; }
.md-toc-h3 .md-toc-inner { margin-left: 4em; }
.md-toc-h4 .md-toc-inner { margin-left: 6em; }
.md-toc-h5 .md-toc-inner { margin-left: 8em; }
.md-toc-h6 .md-toc-inner { margin-left: 10em; }
@media screen and (max-width: 48em) {
  .md-toc-h3 .md-toc-inner { margin-left: 3.5em; }
  .md-toc-h4 .md-toc-inner { margin-left: 5em; }
  .md-toc-h5 .md-toc-inner { margin-left: 6.5em; }
  .md-toc-h6 .md-toc-inner { margin-left: 8em; }
}
a.md-toc-inner { font-size: inherit; font-style: inherit; font-weight: inherit; line-height: inherit; }
.footnote-line a:not(.reversefootnote) { color: inherit; }
.md-attr { display: none; }
.md-fn-count::after { content: "."; }
code, pre, samp, tt { font-family: var(--monospace); }
kbd { margin: 0px 0.1em; padding: 0.1em 0.6em; font-size: 0.8em; color: rgb(36, 39, 41); background: rgb(255, 255, 255); border: 1px solid rgb(173, 179, 185); border-radius: 3px; box-shadow: rgba(12, 13, 14, 0.2) 0px 1px 0px, rgb(255, 255, 255) 0px 0px 0px 2px inset; white-space: nowrap; vertical-align: middle; }
.md-comment { color: rgb(162, 127, 3); opacity: 0.8; font-family: var(--monospace); }
code { text-align: left; vertical-align: initial; }
a.md-print-anchor { white-space: pre !important; border-width: initial !important; border-style: none !important; border-color: initial !important; display: inline-block !important; position: absolute !important; width: 1px !important; right: 0px !important; outline: 0px !important; background: 0px 0px !important; text-decoration: initial !important; text-shadow: initial !important; }
.md-inline-math .MathJax_SVG .noError { display: none !important; }
.html-for-mac .inline-math-svg .MathJax_SVG { vertical-align: 0.2px; }
.md-fences-math .MathJax_SVG_Display, .md-math-block .MathJax_SVG_Display { text-align: center; margin: 0px; position: relative; text-indent: 0px; max-width: none; max-height: none; min-height: 0px; min-width: 100%; width: auto; overflow-y: visible; display: block !important; }
.MathJax_SVG_Display, .md-inline-math .MathJax_SVG_Display { width: auto; margin: inherit; display: inline-block !important; }
.MathJax_SVG .MJX-monospace { font-family: var(--monospace); }
.MathJax_SVG .MJX-sans-serif { font-family: sans-serif; }
.MathJax_SVG { display: inline; font-style: normal; font-weight: 400; line-height: normal; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; zoom: 90%; }
#math-inline-preview-content { zoom: 1.1; }
.MathJax_SVG * { transition: none 0s ease 0s; }
.MathJax_SVG_Display svg { vertical-align: middle !important; margin-bottom: 0px !important; margin-top: 0px !important; }
.os-windows.monocolor-emoji .md-emoji { font-family: "Segoe UI Symbol", sans-serif; }
.md-diagram-panel > svg { max-width: 100%; }
[lang="flow"] svg, [lang="mermaid"] svg { max-width: 100%; height: auto; }
[lang="mermaid"] .node text { font-size: 1rem; }
table tr th { border-bottom: 0px; }
video { max-width: 100%; display: block; margin: 0px auto; }
iframe { max-width: 100%; width: 100%; border: none; }
.highlight td, .highlight tr { border: 0px; }
mark { background: rgb(255, 255, 0); color: rgb(0, 0, 0); }
.md-html-inline .md-plain, .md-html-inline strong, mark .md-inline-math, mark strong { color: inherit; }
.md-expand mark .md-meta { opacity: 0.3 !important; }
mark .md-meta { color: rgb(0, 0, 0); }
@media print {
  .typora-export h1, .typora-export h2, .typora-export h3, .typora-export h4, .typora-export h5, .typora-export h6 { break-inside: avoid; }
}
.md-diagram-panel .messageText { stroke: none !important; }
.md-diagram-panel .start-state { fill: var(--node-fill); }
.md-diagram-panel .edgeLabel rect { opacity: 1 !important; }
.md-require-zoom-fix foreignobject { font-size: var(--mermaid-font-zoom); }
.md-fences.md-fences-math { font-size: 1em; }
.md-fences-math .MathJax_SVG_Display { margin-top: 8px; cursor: default; }
.md-fences-advanced:not(.md-focus) { padding: 0px; white-space: nowrap; border: 0px; }
.md-fences-advanced:not(.md-focus) { background: inherit; }
.typora-export-show-outline .typora-export-content { max-width: 1440px; margin: auto; display: flex; flex-direction: row; }
.typora-export-sidebar { width: 300px; font-size: 0.8rem; margin-top: 80px; margin-right: 18px; }
.typora-export-show-outline #write { --webkit-flex:2; flex: 2 1 0%; }
.typora-export-sidebar .outline-content { position: fixed; top: 0px; max-height: 100%; overflow: hidden auto; padding-bottom: 30px; padding-top: 60px; width: 300px; }
@media screen and (max-width: 1024px) {
  .typora-export-sidebar, .typora-export-sidebar .outline-content { width: 240px; }
}
@media screen and (max-width: 800px) {
  .typora-export-sidebar { display: none; }
}
.outline-content li, .outline-content ul { margin-left: 0px; margin-right: 0px; padding-left: 0px; padding-right: 0px; list-style: none; }
.outline-content ul { margin-top: 0px; margin-bottom: 0px; }
.outline-content strong { font-weight: 400; }
.outline-expander { width: 1rem; height: 1.42857rem; position: relative; display: table-cell; vertical-align: middle; cursor: pointer; padding-left: 4px; }
.outline-expander::before { content: ""; position: relative; font-family: Ionicons; display: inline-block; font-size: 8px; vertical-align: middle; }
.outline-item { padding-top: 3px; padding-bottom: 3px; cursor: pointer; }
.outline-expander:hover::before { content: ""; }
.outline-h1 > .outline-item { padding-left: 0px; }
.outline-h2 > .outline-item { padding-left: 1em; }
.outline-h3 > .outline-item { padding-left: 2em; }
.outline-h4 > .outline-item { padding-left: 3em; }
.outline-h5 > .outline-item { padding-left: 4em; }
.outline-h6 > .outline-item { padding-left: 5em; }
.outline-label { cursor: pointer; display: table-cell; vertical-align: middle; text-decoration: none; color: inherit; }
.outline-label:hover { text-decoration: underline; }
.outline-item:hover { border-color: rgb(245, 245, 245); background-color: var(--item-hover-bg-color); }
.outline-item:hover { margin-left: -28px; margin-right: -28px; border-left: 28px solid transparent; border-right: 28px solid transparent; }
.outline-item-single .outline-expander::before, .outline-item-single .outline-expander:hover::before { display: none; }
.outline-item-open > .outline-item > .outline-expander::before { content: ""; }
.outline-children { display: none; }
.info-panel-tab-wrapper { display: none; }
.outline-item-open > .outline-children { display: block; }
.typora-export .outline-item { padding-top: 1px; padding-bottom: 1px; }
.typora-export .outline-item:hover { margin-right: -8px; border-right: 8px solid transparent; }
.typora-export .outline-expander::before { content: "+"; font-family: inherit; top: -1px; }
.typora-export .outline-expander:hover::before, .typora-export .outline-item-open > .outline-item > .outline-expander::before { content: "−"; }
.typora-export-collapse-outline .outline-children { display: none; }
.typora-export-collapse-outline .outline-item-open > .outline-children, .typora-export-no-collapse-outline .outline-children { display: block; }
.typora-export-no-collapse-outline .outline-expander::before { content: "" !important; }
.typora-export-show-outline .outline-item-active > .outline-item .outline-label { font-weight: 700; }


.CodeMirror { height: auto; }
.CodeMirror.cm-s-inner { background: inherit; }
.CodeMirror-scroll { overflow: auto hidden; z-index: 3; }
.CodeMirror-gutter-filler, .CodeMirror-scrollbar-filler { background-color: rgb(255, 255, 255); }
.CodeMirror-gutters { border-right: 1px solid rgb(221, 221, 221); background: inherit; white-space: nowrap; }
.CodeMirror-linenumber { padding: 0px 3px 0px 5px; text-align: right; color: rgb(153, 153, 153); }
.cm-s-inner .cm-keyword { color: rgb(119, 0, 136); }
.cm-s-inner .cm-atom, .cm-s-inner.cm-atom { color: rgb(34, 17, 153); }
.cm-s-inner .cm-number { color: rgb(17, 102, 68); }
.cm-s-inner .cm-def { color: rgb(0, 0, 255); }
.cm-s-inner .cm-variable { color: rgb(0, 0, 0); }
.cm-s-inner .cm-variable-2 { color: rgb(0, 85, 170); }
.cm-s-inner .cm-variable-3 { color: rgb(0, 136, 85); }
.cm-s-inner .cm-string { color: rgb(170, 17, 17); }
.cm-s-inner .cm-property { color: rgb(0, 0, 0); }
.cm-s-inner .cm-operator { color: rgb(152, 26, 26); }
.cm-s-inner .cm-comment, .cm-s-inner.cm-comment { color: rgb(170, 85, 0); }
.cm-s-inner .cm-string-2 { color: rgb(255, 85, 0); }
.cm-s-inner .cm-meta { color: rgb(85, 85, 85); }
.cm-s-inner .cm-qualifier { color: rgb(85, 85, 85); }
.cm-s-inner .cm-builtin { color: rgb(51, 0, 170); }
.cm-s-inner .cm-bracket { color: rgb(153, 153, 119); }
.cm-s-inner .cm-tag { color: rgb(17, 119, 0); }
.cm-s-inner .cm-attribute { color: rgb(0, 0, 204); }
.cm-s-inner .cm-header, .cm-s-inner.cm-header { color: rgb(0, 0, 255); }
.cm-s-inner .cm-quote, .cm-s-inner.cm-quote { color: rgb(0, 153, 0); }
.cm-s-inner .cm-hr, .cm-s-inner.cm-hr { color: rgb(153, 153, 153); }
.cm-s-inner .cm-link, .cm-s-inner.cm-link { color: rgb(0, 0, 204); }
.cm-negative { color: rgb(221, 68, 68); }
.cm-positive { color: rgb(34, 153, 34); }
.cm-header, .cm-strong { font-weight: 700; }
.cm-del { text-decoration: line-through; }
.cm-em { font-style: italic; }
.cm-link { text-decoration: underline; }
.cm-error { color: red; }
.cm-invalidchar { color: red; }
.cm-constant { color: rgb(38, 139, 210); }
.cm-defined { color: rgb(181, 137, 0); }
div.CodeMirror span.CodeMirror-matchingbracket { color: rgb(0, 255, 0); }
div.CodeMirror span.CodeMirror-nonmatchingbracket { color: rgb(255, 34, 34); }
.cm-s-inner .CodeMirror-activeline-background { background: inherit; }
.CodeMirror { position: relative; overflow: hidden; }
.CodeMirror-scroll { height: 100%; outline: 0px; position: relative; box-sizing: content-box; background: inherit; }
.CodeMirror-sizer { position: relative; }
.CodeMirror-gutter-filler, .CodeMirror-hscrollbar, .CodeMirror-scrollbar-filler, .CodeMirror-vscrollbar { position: absolute; z-index: 6; display: none; }
.CodeMirror-vscrollbar { right: 0px; top: 0px; overflow: hidden; }
.CodeMirror-hscrollbar { bottom: 0px; left: 0px; overflow: hidden; }
.CodeMirror-scrollbar-filler { right: 0px; bottom: 0px; }
.CodeMirror-gutter-filler { left: 0px; bottom: 0px; }
.CodeMirror-gutters { position: absolute; left: 0px; top: 0px; padding-bottom: 30px; z-index: 3; }
.CodeMirror-gutter { white-space: normal; height: 100%; box-sizing: content-box; padding-bottom: 30px; margin-bottom: -32px; display: inline-block; }
.CodeMirror-gutter-wrapper { position: absolute; z-index: 4; background: 0px 0px !important; border: none !important; }
.CodeMirror-gutter-background { position: absolute; top: 0px; bottom: 0px; z-index: 4; }
.CodeMirror-gutter-elt { position: absolute; cursor: default; z-index: 4; }
.CodeMirror-lines { cursor: text; }
.CodeMirror pre { border-radius: 0px; border-width: 0px; background: 0px 0px; font-family: inherit; font-size: inherit; margin: 0px; white-space: pre; overflow-wrap: normal; color: inherit; z-index: 2; position: relative; overflow: visible; }
.CodeMirror-wrap pre { overflow-wrap: break-word; white-space: pre-wrap; word-break: normal; }
.CodeMirror-code pre { border-right: 30px solid transparent; width: fit-content; }
.CodeMirror-wrap .CodeMirror-code pre { border-right: none; width: auto; }
.CodeMirror-linebackground { position: absolute; left: 0px; right: 0px; top: 0px; bottom: 0px; z-index: 0; }
.CodeMirror-linewidget { position: relative; z-index: 2; overflow: auto; }
.CodeMirror-wrap .CodeMirror-scroll { overflow-x: hidden; }
.CodeMirror-measure { position: absolute; width: 100%; height: 0px; overflow: hidden; visibility: hidden; }
.CodeMirror-measure pre { position: static; }
.CodeMirror div.CodeMirror-cursor { position: absolute; visibility: hidden; border-right: none; width: 0px; }
.CodeMirror div.CodeMirror-cursor { visibility: hidden; }
.CodeMirror-focused div.CodeMirror-cursor { visibility: inherit; }
.cm-searching { background: rgba(255, 255, 0, 0.4); }
span.cm-underlined { text-decoration: underline; }
span.cm-strikethrough { text-decoration: line-through; }
.cm-tw-syntaxerror { color: rgb(255, 255, 255); background-color: rgb(153, 0, 0); }
.cm-tw-deleted { text-decoration: line-through; }
.cm-tw-header5 { font-weight: 700; }
.cm-tw-listitem:first-child { padding-left: 10px; }
.cm-tw-box { border-style: solid; border-right-width: 1px; border-bottom-width: 1px; border-left-width: 1px; border-color: inherit; border-top-width: 0px !important; }
.cm-tw-underline { text-decoration: underline; }
@media print {
  .CodeMirror div.CodeMirror-cursor { visibility: hidden; }
}


:root {
    --side-bar-bg-color: #fafafa;
    --control-text-color: #777;
}

@include-when-export url(https://fonts.loli.net/css?family=Open+Sans:400italic,700italic,700,400&subset=latin,latin-ext);

/* open-sans-regular - latin-ext_latin */
  /* open-sans-italic - latin-ext_latin */
    /* open-sans-700 - latin-ext_latin */
    /* open-sans-700italic - latin-ext_latin */
  html {
    font-size: 16px;
}

body {
    font-family: "Open Sans","Clear Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
    color: rgb(51, 51, 51);
    line-height: 1.6;
}

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</style><title>02OC语言</title>
</head>
<body class='typora-export os-windows'><div class='typora-export-content'>
<div id='write'  class=''><h2 id='二oc语言相关'><span>二.OC语言相关</span></h2><h3 id='21-分类category'><span>2.1 分类Category</span></h3><p><strong><span>A.什么是分类Category？你用分类都做了哪些事情？</span></strong>
<span>OC的</span><code>Category</code><span>是对</span><strong><span>装饰模式</span></strong><span>的一种具体实现。它的主要作用是在不改变原有类的前提下，动态地给这个类添加一些方法、属性、协议等。使用Category做过下面的事情：</span></p><ul><li><span>声明私有方法</span></li><li><span>分解，解耦体积庞大的类文件</span></li><li><span>把Framework的私有方法公开</span></li><li><span>拓展一些快捷的方法</span></li></ul><p><strong><span>B.分类有哪些特点</span></strong>
<span>1.分类最大的特点就是</span><strong><span>运行时决议</span></strong><span>，2.可以为系统类添加分类，</span></p><p><strong><span>C.分类都可以添加哪些内容</span></strong>
<span>实例方法</span>
<span>类方法</span>
<span>协议</span>
<span>属性(只是声明了get方法和set方法，并没有添加实例变量，实例变量需要使用runtime的关联属性)</span></p><p><strong><span>D.分类的几个关键词总结</span></strong>
<span>1.分类添加的方法可以&quot;覆盖&quot;原类方法，(这里的覆盖是指分类方法内存靠前，会最先被查找到，造成效果上是覆盖)</span>
<span>2.同名 分类方法 谁能生效，取决于编译顺序</span>
<span>3.名字相同的分类，会引起编译报错</span></p><p><strong><span>E.请简述分类实现原理</span></strong>
<span>所有的OC类和对象，在Runtime层都是用</span><code>struct</code><span>表示的，Category也不例外，在Runtime层，Category用结构体</span><code>category_t</code><span>表示：</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">struct</span> <span class="cm-def">category_t</span> {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-keyword">const</span> <span class="cm-variable-3">char</span> <span class="cm-variable-3">*</span><span class="cm-variable">name</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">classref_t</span> <span class="cm-variable">cls</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-keyword">struct</span> <span class="cm-def">method_list_t</span> <span class="cm-operator">*</span><span class="cm-variable">instanceMethods</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-keyword">struct</span> <span class="cm-def">method_list_t</span> <span class="cm-operator">*</span><span class="cm-variable">classMethods</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-keyword">struct</span> <span class="cm-def">protocol_list_t</span> <span class="cm-operator">*</span><span class="cm-variable">protocols</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-keyword">struct</span> <span class="cm-def">property_list_t</span> <span class="cm-operator">*</span><span class="cm-variable">instanceProperties</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">};</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 184px;"></div><div class="CodeMirror-gutters" style="display: none; height: 184px;"></div></div></div></pre><p><span>从Category的定义中可以看出来，Category可以添加实例方法，类方法；可以遵守协议，添加属性；但无法添加实例变量。在程序运行的时候，Runtime会将一个类所有的Category的数据，合并到类信息中（类对象、元类对象中）</span></p><p><strong><span>注意：</span></strong><span> 在Category中可以有属性（property），但是该属性只是生成了getter和setter方法的声明，并没有产生对应的实现，更不会添加对应的实例变量。如果想为实例对象添加实例变量，可以使用</span><strong><span>关联对象技术</span></strong><span>。</span></p><p><span>分类的属性，只是生成了getter和setter方法的声明，并没有产生对应的实现，正确的理解就是：只在.h文件有方法定义，但是在.m文件没有对应的实现，所以调用依旧会报经典错误：</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> unrecognized selector sent to instance xxx</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 23px;"></div><div class="CodeMirror-gutters" style="display: none; height: 23px;"></div></div></div></pre><p><strong><span>F.分类的对象方法，类方法都存在哪里？</span></strong>
<span>一个类的所有分类的 对象方法放在类对象中，所有分类的类方法存放在元类中。可以通过阅读Runtime源码得知：通过runtime动态将分类的方法合并到类对象、元类对象中的。</span></p><p><strong><span>G.分类的方法是如何添加到类对象方法列表中的？</span></strong>
<span>通过阅读Runtime源码得知大概流程是：</span>
<span>1.获取分类列表的count，然后原来的类方法列表内存移动count</span>
<span>2.分类列表内存拷贝到原来的类方法列表的</span><strong><span>前方</span></strong>
<span>3.所以调用同名的方法，优先调用分类的方法</span>
<span>4.分类具有同名的方法，根据编译顺序决定，取最后编译分类的方法列表</span></p><p><strong><span>H.Category的加载处理过程？</span></strong>
<span>1.通过Runtime加载某个类的所有Category数据</span>
<span>2.把所有Category的方法、属性、协议数据，合并到一个大数组中</span>
<span>3.后面参与编译的Category数据，会在数组的前面</span>
<span>4.将合并后的分类数据（方法、属性、协议），插入到类原来数据的前面</span></p><p><strong><span>J.Category和Class Extension的区别是什么？</span></strong>
<span>Class Extension在编译的时候，它的数据就已经包含在类信息中</span>
<span>Category是在运行时，才会将数据合并到类信息中.(也就是运行时决议)</span></p><p><code>Extension</code><span>一般隐藏类的私有信息，你必须有一个类才能为这个类添加 </span><code>Extension</code><span>，所以你无法为系统的类比如</span><code>NSString</code><span>添加</span><code>Extension</code><span>。</span>
<code>Category</code><span>却可以为系统类添加分类。</span></p><p><code>Extension</code><span>可以添加成员变量，而</span><code>Category</code><span>则不可以。</span></p><p><strong><span>K.Category中有load方法吗？load方法是什么时候调用的？load 方法能继承吗？</span></strong>
<span>1.有load方法</span>
<span>2.每个类、分类的+load，在程序运行过程中只调用一次，只有在加载类时候调用一次</span>
<span>3.不存在分类的+load方法覆盖类的+load方法</span>
<span>4.load方法可以继承，但是一般情况下不会主动去调用load方法，都是让系统自动调用</span></p><p><strong><span>L.+initialize方法是怎么调用的？</span></strong>
<code>+initialize</code><span>方法会在类第一次接收到消息时调用，消息转发机制调用的（objc_send）。调用的顺序是：先调用父类的+initialize，再调用子类的+initialize；(先初始化父类，再初始化子类，每个类只会初始化1次）</span></p><p><strong><span>Q.+initialize和+load的很大区别是？</span></strong>
<span>1.调用方式</span></p><ul><li><span>1&gt; </span><code>+load</code><span>是根据函数地址直接调用</span></li><li><span>2&gt; </span><code>+initialize</code><span>是通过</span><code>objc_msgSend</code><span>调用</span></li></ul><p><span>2.调用时刻</span></p><ul><li><span>1&gt; </span><code>load</code><span>是Runtime加载类、分类的时候调用（只会调用1次）</span></li><li><span>2&gt; </span><code>+initialize</code><span>是类第一次接收到消息的时候调用，每一个类只会</span><code>initialize</code><span>一次（父类的initialize方法可能会被调用多次），如果一个类一直没用被用到，那么它的</span><code>+initialize</code><span>可能会一直不会被调用。</span></li></ul><p><span>3.如果分类实现了</span><code>+initialize</code><span>，就覆盖类本身的</span><code>+initialize</code><span>调用</span>
<span>4.</span><code>+load</code><span>是通过函数指针指向函数，拿到函数地址，分开来直接调用的，直接通过内存地址查找调用的。</span>
<span>5.</span><code>load</code><span>和</span><code>initialize</code><span>方法都会在实例化对象之前调用，以</span><code>main</code><span>函数为分水岭，前者在</span><code>main</code><span>函数之前调用，后者在之后调用。这两个方法会被自动调用，不能手动调用它们。</span>
<span>6.</span><code>load</code><span>和</span><code>initialize</code><span>方法内部使用了锁，因此它们是线程安全的。实现时要尽可能保持简单，避免阻塞线程，不要再使用锁.</span></p><p><strong><span>W.+initialize和+load的使用场景？</span></strong>
<span>1.对于</span><code>+load</code><span>方法，常用的一个场景就是在</span><code>+load</code><span>方法里面进行</span><code>method Swizzling</code><span>方法交换。</span>
<span>2.</span><code>initialize</code><span>方法主要用来对一些不方便在编译期初始化的对象进行赋值，例如用于初始化全局变量或静态变量。代码:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="objc"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="objc"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">// In Parent.m</span></span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">static</span> <span class="cm-variable-3">int</span> <span class="cm-variable">someNumber</span> <span class="cm-operator">=</span> <span class="cm-number">0</span>; &nbsp; &nbsp; <span class="cm-comment">// int类型可以在编译期赋值</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">static</span> <span class="cm-variable">NSMutableArray</span> <span class="cm-operator">*</span><span class="cm-variable">someObjects</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-operator">+</span> (<span class="cm-variable-3">void</span>)<span class="cm-variable">initialize</span> {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-keyword">if</span> (<span class="cm-keyword">self</span> <span class="cm-operator">==</span> [<span class="cm-variable">Parent</span> <span class="cm-variable">class</span>]) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp; &nbsp; &nbsp;<span class="cm-comment">// 不方便编译期复制的对象在这里赋值</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp; &nbsp; &nbsp;<span class="cm-variable">someObjects</span> <span class="cm-operator">=</span> [[<span class="cm-variable">NSMutableArray</span> <span class="cm-variable">alloc</span>] <span class="cm-variable">init</span>];</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 230px;"></div><div class="CodeMirror-gutters" style="display: none; height: 230px;"></div></div></div></pre><h3 id='22-关联对象'><span>2.2 关联对象</span></h3><p><strong><span>A.能否给分类添加&quot;成员变量&quot;？</span></strong>
<span>答：不能为Category分类添加成员变量，不过可以使用</span><strong><span>关联对象技术</span></strong><span>来达到添加成员变量的效果。</span></p><p><span>Runtime关联对象，核心函数:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">id</span> <span class="cm-def">objc_getAssociatedObject</span>(<span class="cm-variable">id</span> <span class="cm-variable">object</span>, <span class="cm-keyword">const</span> <span class="cm-variable-3">void</span> <span class="cm-variable-3">*</span> <span class="cm-variable">key</span>)</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable-3">void</span> <span class="cm-def">objc_setAssociatedObject</span>(<span class="cm-variable">id</span> <span class="cm-variable">object</span>, <span class="cm-keyword">const</span> <span class="cm-variable-3">void</span> <span class="cm-variable-3">*</span> <span class="cm-variable">key</span>, <span class="cm-variable">id</span> <span class="cm-variable">value</span>, <span class="cm-variable">objc_AssociationPolicy</span> <span class="cm-variable">policy</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable-3">void</span> <span class="cm-def">objc_removeAssociatedObjects</span>(<span class="cm-variable">id</span> <span class="cm-variable">object</span>)</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 92px;"></div><div class="CodeMirror-gutters" style="display: none; height: 92px;"></div></div></div></pre><p><strong><span>B.通过关联对象添加的&quot;成员变量&quot;，添加到了哪里？</span></strong>
<span>答: 没有添加到宿主类上，存储了一个全局的容器里，见下面问题:</span></p><p><strong><span>C.关联对象的本质？</span></strong>
<span>1.关联对象由</span><code>AssociationsManager</code><span>管理并在</span><code>AssociationsHashMap</code><span>存储</span>
<span>2.所有对象的关联内容，都在</span><strong><span>同一个全局容器</span></strong></p><p><span>3.</span><code>AssociationsManager</code><span>内部有一个</span><code>AssociationsHashMap</code><span>的对象（可以看做字典）</span>
<span>4.</span><code>AssociationsHashMap</code><span>字典的key是传过来的object，value是</span><code>ObjectAssociationMap</code><span>对象（可以看做字典）</span>
<span>5.</span><code>ObjectAssociationMap</code><span>字典的key是传过来的key，value是</span><code>ObjcAssociation</code>
<span>6.</span><code>ObjcAssociation</code><span>中存放者value和policy</span></p><p><strong><span>D.关联对象是否储存在类对象内存中？</span></strong>
<span>关联对象并不是存储在被关联对象本身内存中，关联对象存储在全局的统一的一个AssociationsManager，AssociationsHashMap中</span></p><p><strong><span>E.设置关联对象为nil，会发生什么？</span></strong>
<span>相当于是移除关联对象，内部会有一个</span><code>erase</code><span>(移除)操作。</span></p><h3 id='23-扩展extension'><span>2.3 扩展(Extension)</span></h3><p><strong><span>A.你用扩展都做了哪些事情？</span></strong>
<span>1.声明私有属性</span>
<span>2.声明私有方法</span>
<span>3.声明私有成员变量</span></p><p><strong><span>B.扩展有哪些特点</span></strong>
<span>1.编译时决议。（既编译时就已经确定了内存布局）</span>
<span>2.只以声明的形式存在，多数情况下寄生于宿主类的.m中</span>
<span>3.不能为系统类添加扩展</span></p><h3 id='24-代理delegate'><span>2.4 代理(Delegate)</span></h3><p><span>代理(Delegate)准确的来说是一种软件设计模式，最大的特点是 一对一传递数据</span>
<span>1.声明协议</span><code>@protocol</code>
<span>2.设置委托方</span><code>delegate=xx</code><span>，一般声明为</span><code>weak</code><span>来避免循环引用</span></p><h3 id='25-通知nsnotification'><span>2.5 通知(NSNotification)</span></h3><p><span>通知是使用</span><strong><span>观察者模式</span></strong><span>来实现的用于跨层传递消息的机制，是一对多的传递方式。</span></p><h3 id='26-kvo'><span>2.6 KVO</span></h3><p><a href='https://www.jianshu.com/p/5477cf91bb32' target='_blank' class='url'>https://www.jianshu.com/p/5477cf91bb32</a></p><p><strong><span>A.什么是KVO？</span></strong>
<span>KVO是</span><code>key-value observing</code><span>的缩写，它是OC对观察者模式的又一实现，俗称“键值监听”；可以监听对象某个属性值的变化。Apple使用了isa混写（isa-swizzling）来实现KVO.</span></p><p><strong><span>B.KVO的实现细节和原理？</span></strong>
<span>当我们调用</span><code>addObserver:forKeyPath</code><span>方法来注册观察者A时，系统会在运行时，创建</span><code>NSKVONotifying_A</code><span>的子类，并重写要监听属性的set方法，并将类A的isa指针指向</span><code>NSKVONotifying_A</code><span>，而在这个子类的set方法内部，会顺序调用</span><code>willChangeValueForKey</code><span>方法，原来的setter方法实现，和</span><code>didChangeValueForKey</code><span>方法。而</span><code>didChangeValueForKey</code><span>方法内部又会调用监听器的</span><code>observeValueForKeyPath:ofObject:change:context:</code><span>监听方法。</span></p><p><span>可以通过下面来证明有子类的场景:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="shell"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="shell"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-def">$ po</span> object_getClassName(obj)</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 23px;"></div><div class="CodeMirror-gutters" style="display: none; height: 23px;"></div></div></div></pre><p><strong><span>C.通过KVC设置value，KVO能否生效？</span></strong>
<span>能生效，这里简述KVC原理，顺便回调，KVO能否生效的关键就是是否走了setter方法。</span></p><p><strong><span>D.通过成员变量设置value，KVO能否生效？</span></strong>
<span>不能生效，因为直接通过成员变量赋值，不能走set方法，所以不会触发KVO</span></p><p><span>结论：</span>
<span>1.使用setter方法改变值，KVO才会生效</span>
<span>2.使用</span><code>setValue:forKey:</code><span>改变值，KVO会生效</span>
<span>3.成员变量直接修改，需要</span><strong><span>手动添加</span></strong><span>KVO才会生效，手动触发KVO，下面两个方法都需要调用，并且这两个方法缺一不可。</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">[<span class="cm-variable">self</span> <span class="cm-variable">willChangeValueForKey</span>:<span class="cm-variable">@</span><span class="cm-string">"value"</span>];</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">_value</span> <span class="cm-operator">+=</span> <span class="cm-number">1</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">[<span class="cm-variable">self</span> <span class="cm-variable">didChangeValueForKey</span>:<span class="cm-variable">@</span><span class="cm-string">"value"</span>];</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 69px;"></div><div class="CodeMirror-gutters" style="display: none; height: 69px;"></div></div></div></pre><h3 id='27-kvc'><span>2.7 KVC</span></h3><p><strong><span>A.什么是KVC？</span></strong>
<span>KVC是</span><code>key-value coding</code><span>的缩写，一般称为键值编码，可以允许开发者通过Key名直接访问对象的属性，或者给对象的属性赋值。不过需要注意的是KVC是不符合面向对象思想的，因为可以给私有属性赋值。</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="objc"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="objc"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">// 直接通过Key来取值</span></span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-operator">-</span> (<span class="cm-variable">nullable</span> <span class="cm-keyword">id</span>)<span class="cm-variable">valueForKey</span>:(<span class="cm-variable">NSString</span> <span class="cm-operator">*</span>)<span class="cm-variable">key</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">// 通过Key来设值</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-operator">-</span> (<span class="cm-variable-3">void</span>)<span class="cm-variable">setValue</span>:(<span class="cm-variable">nullable</span> <span class="cm-keyword">id</span>)<span class="cm-variable">value</span> <span class="cm-variable">forKey</span>:(<span class="cm-variable">NSString</span> <span class="cm-operator">*</span>)<span class="cm-variable">key</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">// 通过KeyPath来取值</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-operator">-</span> (<span class="cm-variable">nullable</span> <span class="cm-keyword">id</span>)<span class="cm-variable">valueForKeyPath</span>:(<span class="cm-variable">NSString</span> <span class="cm-operator">*</span>)<span class="cm-variable">keyPath</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">// 通过KeyPath来设值</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-operator">-</span> (<span class="cm-variable-3">void</span>)<span class="cm-variable">setValue</span>:(<span class="cm-variable">nullable</span> <span class="cm-keyword">id</span>)<span class="cm-variable">value</span> <span class="cm-variable">forKeyPath</span>:(<span class="cm-variable">NSString</span> <span class="cm-operator">*</span>)<span class="cm-variable">keyPath</span>;</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 184px;"></div><div class="CodeMirror-gutters" style="display: none; height: 184px;"></div></div></div></pre><p><strong><span>B.KVC实现原理？</span></strong>
<a href='https://www.jianshu.com/p/45cbd324ea65' target='_blank' class='url'>https://www.jianshu.com/p/45cbd324ea65</a></p><p><span>1.赋值时首先判断有没有对应的set方法，如果有直接调用set方法</span>
<span>2.如果没有set方法，查看有没有和key一样的成员变量，如果有，直接赋值</span>
<span>3.如果没有成员变量，查找有没有对应的属性，如果有，直接赋值</span>
<span>4.如果都没有，调用</span><code>setvalue:forUndefinedKey:</code><span>方法</span></p><h3 id='28-属性关键字'><span>2.8 属性关键字</span></h3><p><span>读写权限：</span></p><ul><li><code>readonly</code></li><li><code>readwrite</code><span>  // 默认的关键字</span></li></ul><p><span>原子性:</span></p><ul><li><code>atomic</code></li><li><code>nonatomic</code></li></ul><p><span>比如</span><code>atomic</code><span>修饰属性是Array，那么对Array的赋值和获取，是可以线程安全的，但是Array本身方法(添加/删除元素)的操作，是无法保证线程安全的。</span></p><p><span>引用计数：</span></p><ul><li><code>retain</code><span>/</span><code>strong</code></li><li><code>assign</code></li><li><code>weak</code></li><li><code>copy</code></li></ul><p><strong><span>A.assign和weak的区别？</span></strong>
<span>1.</span><code>assign</code><span>既可以修饰基本数据类型，也可以修饰对象。而</span><code>weak</code><span>只能修饰对象</span>
<span>2.</span><code>assign</code><span>和</span><code>weak</code><span>修饰对象时，都会不改变其引用计数。</span>
<span>3.</span><code>assign</code><span>修饰的对象被释放后，仍然指向对象的地址，产生悬垂指针，调用就会野指针崩溃</span>
<span>4.</span><code>weak</code><span>修饰的对象在被释放后，会自动设置</span><code>nil</code></p><p><strong><span>B.copy和mutableCopy的区别？</span></strong></p><p><span>浅拷贝就是内存地址的复制，让目标对象指针和源对象指向</span><strong><span>同一片</span></strong><span>内存空间。</span>
<span>深拷贝让目标对象指针和源对象指针，指向</span><strong><span>两片</span></strong><span>内容相同的内存空间</span></p><p><span>深拷贝不会增加引用计数</span>
<span>浅拷贝会增加引用计数</span>
<span>深拷贝会开辟新的内存空间，浅拷贝不会开辟新的内存空间，这里内存空间一般是指堆空间</span></p><blockquote><p><span>注：Swift里面就没有深浅拷贝问题，因为Swift大多数使用的是值类型struct</span></p></blockquote><figure><table><thead><tr><th><span>源对象类型</span></th><th><span>拷贝方式</span></th><th><span>目标对象类型</span></th><th><span>拷贝类型</span></th></tr></thead><tbody><tr><td><span>mutable对象</span></td><td><span>copy</span></td><td><span>不可变</span></td><td><span>深拷贝</span></td></tr><tr><td><span>mutable对象</span></td><td><span>mutableCopy</span></td><td><span>可变</span></td><td><span>深拷贝</span></td></tr><tr><td><span>immutable对象</span></td><td><span>copy</span></td><td><span>不可变</span></td><td><span>浅拷贝</span></td></tr><tr><td><span>immutable对象</span></td><td><span>mutableCopy</span></td><td><span>可变</span></td><td><span>深拷贝</span></td></tr></tbody></table></figure><p><span>可变对象的copy和mutableCopy都是深拷贝</span>
<span>不可变对象的copy是浅拷贝，mutableCopy是深拷贝</span>
<strong><span>copy方法返回的都是不可变对象</span></strong><span> (对于OC，这句话很重要)</span></p><p><strong><span>C.copy修饰</span><code>NSMutableArray</code><span>有什么问题？</span></strong>
<span>会导致一些不可预测的异常，因为</span>
<span>如果赋值过来的是</span><code>NSMutableArray</code><span>，copy之后是</span><code>NSArray</code><span>.</span></p><p><strong><span>D.属性</span><code>NSString</code><span>经常使用copy关键字，为什么？</span></strong></p><p><span>属性</span><code>NSString</code><span>（或</span><code>NSArray</code><span>，</span><code>NSDictionary</code><span>）经常使用copy关键字是因为了让本对象的属性不受外界影响,使用</span><code>copy</code><span>无论给我传入是一个可变对象还是不可对象,我本身持有的就是一个不可变的副本.</span></p><p><span>我们应该根据实际情况合理的选择修饰符，比如只是简单的字符串赋值的属性，我们使用</span><code>strong</code><span>和</span><code>copy</code><span>修饰，效果是一样的；但是对于涉及到可变字符串的修改赋值的属性，我们一定要使用</span><code>copy</code><span>进行修饰，这样才能保证代码的封装性，否则会产生值被意外修改的bug。</span></p><p><span>在Swift中就不会有这样的问题，因为Swift中</span><code>String</code><span>,</span><code>Array</code><span>等都是值类型，没有引用问题，而且也可以使用</span><code>let</code><span>来保证不可变。</span></p><p><strong><span>如果改用</span><code>strong</code><span>关键字，可能造成什么问题？</span></strong>
<span>如果我们使用是strong,那么这个属性就有可能指向一个可变对象,如果这个可变对象在外部被修改了,那么会影响该属性.</span></p><p><strong><span>D2.属性</span><code>copy</code><span>的使用原理？</span></strong>
<code>copy</code><span>此特质所表达的所属关系与</span><code>strong</code><span>类似。然而setter方法并不保留新值，而是将其“拷贝” (copy)。</span>
<span>当属性类型为</span><code>NSString</code><span>时，经常用此特质来保护其封装性，因为传递给setter方法的新值有可能指向一个</span><code>NSMutableString</code><span>类的实例。此时若是不拷贝字符串，那么设置完属性之后，字符串的值就可能会在对象不知情的情况下遭外界更改。</span></p><p><strong><span>E.MRC下如何重写retain修饰变量的setter方法？</span></strong></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded md-focus" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner CodeMirror-wrap CodeMirror-focused" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 32.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><span><span>​</span>x</span></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="" style="position: relative;"><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">@property</span> (<span class="cm-variable">nonatomic</span> ,<span class="cm-variable">retain</span>) <span class="cm-variable">id</span> <span class="cm-variable">obj</span>;</span></pre></div><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-operator">-</span> (<span class="cm-variable-3">void</span>)<span class="cm-variable">setObj</span>:(<span class="cm-variable">id</span>)<span class="cm-variable">obj</span> {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-keyword">if</span> (<span class="cm-variable">_obj</span> <span class="cm-operator">!=</span> <span class="cm-variable">obj</span>) {</span></pre><div class="" style="position: relative;"><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp; &nbsp;  [<span class="cm-variable">_obj</span> <span class="cm-variable">release</span>]; &nbsp;<span class="cm-comment">// 很多人会忘写这句</span></span></pre></div><div class="" style="position: relative;"><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp; &nbsp; &nbsp;<span class="cm-variable">_obj</span> <span class="cm-operator">=</span> [<span class="cm-variable">obj</span> <span class="cm-variable">retain</span>];</span></pre></div><div class="" style="position: relative;"><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  }</span></pre></div><div class="" style="position: relative;"><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 184px;"></div><div class="CodeMirror-gutters" style="display: none; height: 184px;"></div></div></div></pre><p>&nbsp;</p></div></div>
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